About a three feet long piece of old steel pipe , bolt a big hinge to one end, drill an eye-bolt to the other end. Fasten the hinge to the top of your elevator then run a small rope through the eyelet through a pulley and back down to a location next to the elevator.
A quick yank on the rope lifts the pipe, let it slam back down to make your noise, if this isn't loud enough or doesn't sound quite right to you experment by loosely wiring metal junk/objects to the pipe.
Having the pipe hinged and attached keeps the whole thing more predictablly located.